Commutation of contact converters



Jan. 20, 1959 M. wlDAKowlcH 2,870,394

coMuuTATIoN oF CONTACT coNvERTERs Filed May 9, 1955 H2 HBL- INVENTOR MARlus wloAKowlcH ATTORNEYs Unite States CGMMUTATION F CNTACT CUNVERTERS Marius Widakowich, Bromma, Sweden Application May 9, 1955, Serial No. 506,989

Claims priority, application Sweden May 14, 1954 4 Claims. (Cl. 321-49) The present invention refers to apparatus adapted to improve the commutation of contact converters for converting direct current to a periodically changing current (alternating current) or vice versa. Fthe problem in converters of this type is to eliect either a currentless or a voltageless interruption and closure of the contacts to provide for a long life of the contacts. According to the present invention low-tension interruption and lowcurrent closure is attained by the inserting of condensers into the alternating current side of the converter and by connecting inductances in series with the contacts of the converter. Electric valves are provided in parallel with the inductances to render possible an equalization of the magnetic energy accumulated in the inductances upon every contact interruption or, breaking, or, the valves may be arranged over the D. C. terminals of the converter in series with condensers for the attainment of the same object in view.

The invention, which is here applied to a contact converter, will be explained more closely with reference to the accompanying drawing wherein, Figs. l, 2, and 3 show different arrangements of the contact converter according to the present invention, and Figs. 4, 5, and 6, show magnetization curve diagrams for the inductance used in the arrangements shown in Figs. 1 3 respectively.

'lhe contact apparatus 1, consists of the working contacts of a so-called vibrator, converts D. C. voltage into A. C. voltage through a reciprocating movement of the oscillating contacts 2 and 3 cooperating with the fixed contacts d, S, if and 7. The D. C. voltage applied across the points 8 and 9 is conveyed to the fixed contacts via the inductance 16. The alternating current load 11 is connected to the alternating current output terminals 2 and 3 of the converter, a condenser i2 being connected in parallel across said terminals. Upon every closure of the contacts of the Contact apparatus only a small current is permitted to pass by reason of the reactance effect of the inductance. lf this inductance were not provided, the condenser 12 would cause a very powerful charging current impulse at every Contact closure; however, such an impulse cannot come about by reason of the inability of the inductance to permit any rapid current changes.

At the breaking of the contacts the condenser 12 shows the same voltage as do the D. C. voltage contacts of the converter during the very tirst moment, so that, if no attention is paid tothe losses due to the resistance of the condenser or the leading-in wires, there will be no difference in voltage at the moment of contact breaking. The current in the inductance lti would of course have a tendency toward continuing upon a completed contact breaking to form electric arcs over the contacts, if the valve 13, such for example as the diode element shown, were not connected in parallel across the inductance. In this Way a continued flow of current is rendered possible, which is reduced incrementally to zero at a rate dependent on the time constant of the circuit -13. A

atent Patented Jari. 20, E95

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condenser 14 may preferably be connected in parallel across the D. C. voltage terminals 8 and 9 to prevent a voltage increase at every moment of contact breaking between the points 8 and 9 should the voltage source or the supply leads of the same possess any appreciable impedance.

Obviously, the ohmic voltage drop across the inductance 10 and the valve 13 should be as small as possible, so that the voltage over the points 4 and 6 will not increase immediately upon contact breaking, for otherwise the voltage over the points 2 and 3 would be less than that over the fixed contacts just left in the contact apparatus. lt may be stated that, if no attention is paid to the losses due to the resistance of the condenser or the leading-in wires, the voltage to be interrupted by the contact apparatus is only constituted by the ohmic voltage drop in the inductance iti and the valve 13. lt might happen that the time constant with a very low ohmic resistance of the circuit 10, 13 is so great that the current has had no time to become sufliciently reduced at the next contact closure. ri"nis would involve that a connecting-in current impulse of the momentary value of the residual current would load the apparatus. On the other hand, it would not be suitable to reduce the time constant of the circuit for instance by an increase of the ohmic resistance, for in that case the differential voltage to be interrupted by the contacts would become greater.

According to the invention, it is possible to employ circuits with a considerably greater time constant, if a scheme according to Fig. 2 is made use of. Here, two inductances are arranged with shunted valves adapted to function alternately. Consequently, the time during which the current can disappear in the circuit l'tl, 13 is not only the duration of one opening interval but the duration of two opening intervals plus the duration of a halfwave.

in Fig. 3, the arrangement is modified on the direct current side in such a manner that the valve 13, which is series-connected to a condenser 15, is shunted across the direct current terminals of the converter. The advantage of this arrangement resides in that the voltage increase produced between the points 4 and 6 upon every contact breaking is only constituted by the voltage drop of the valve and not by the voltage drop of the valve plus the ohmic resistance of the inductance, provided the condenser 15 is sufficiently large. Preferably, either the valve or the condenser, or both of them, are connected in parallel across an impedance permitting the condenser to assume a voltage level corresponding to the D. C. voltage between the points 4 and 6. Here, the condenser may assume the function of the condenser 14, entirely or in part. it is of course also conceivable to combine the arrangement according to Fig. l or Fig. 2 with the arrangement of Fig. 3, so that a valve is connected in parallel to the inductance and another valve in series wi hta condenser is connected in parallel to the D. C. voltage terminals.

There is nothing preventing a reduction of the current strength immediately before every moment of contact breaking by connecting a condenser lo in series with tlz load 11 (as indicated in Fig. 3). The advantage of such an arrangement resides in that not only the differential Voltage over the breaking contacts but also the current strength will be reduced at this critical moment.

The inductance 10 should be provided with an iron core of good quality so as to obtain a magnetization characteristic exhibiting a sharply pronounced knee. Fig. 4 shows two magnetization curves, one exhibiting an ordinary characteristic and the other exhibiting a characteristic having a steep slope and a pronounced knee. The steepness of the last mentioned curve is of importance at the moment of contact closure since it will produce a the load current will pass. The use of iron with a pro-` nounced knee in its characteristic also olers the advantage that the loss of energy in the circuit 10, 13 is consideraoly reduced as will appear from a consideration of Fig. 4, wherein the hatched portion of the diagram above the zero line H represents the energy loss for the mag-l netization curve exhibiting the pronounced knee, 'whereas the hatched portion below the zero line H represents the energy loss for the ordinary magnetization curve. As the magnetization energy in the parallel circuit 10, 13 is converted into heat, it is important that said energy be as small as possible.

It will be found suitable to formthe inductance as a choke with a closed iro-n core, which is so dirnensioned that the linductance value will be highly dependent on the magnetization of the iron so as to be great at the very moment of contact closure, without, however, the energy accumulated in the inductance having to be so great as it would be in case the inductance had a constant value. A certain risk is incurred here, inasmuch as the remanence manifests itself in an undesirable manner by preventing the inductance from attaining its maximum value before every moment of contact closure. This may be obviated by the provision either of a very small air gap in the iron core kor of counter-magnetization either by premagnetization yor by a' permanent magnet connected or built into the circuit. The principle is understood to reside in that the inductance has its maximum value at the very moment of every contact closure,

The circumstances above referred to are illustrated by the curves in Figs. 5 and 6. In Pio. 5, the iron is presumed to have no remanence and the magnetization characteristic is then represented by a single line. When no pre-magnetization of the iron exists, only the magnetic flux p1 is utilized. By use of pre-magnetization, as rep.- l

resented by H2 and H3, a greater flux 412 and p3, respectively, may be utilized. When an iron with remanence is employed, the characteristic will have the ordinary form of a double curve as shown in Fig. 6, wherein the remanence R is indicated by the flux p1. By using a diierent amount of counter-magnetization, as indicated by H2 and H3, a corresponding greater iluX, o2 and o3, respectively, may be utilized.

What I claim is:

l. In a contact converter, Athe combination of a valve connected in parallel to an inductance between one terminal of a continuous voltage source and a fixed contact of the converter, a second valve connected in parallel to a second inductance between said continuous voltage terminal and a second fixed contact of the converter, the connecting points of said groups of valves and inductances adjacent to said continuous voltage terminal being connected over a capacitance to the other terminal or" the continuous voltage source and to a group of two Xed contacts of the converter other than said rst mentio-ned group of lixed contacts.

2. ln a contact converter, the combination of a valve connected in parallel to an inductance between one terminal or a continuous voltage source and a xed contact of the converter, a second valve connected in parallel to a second inductance between said continuous voltage terminal and a second fixed contact .of .the converter, the connecting points of said groups of valves and inductances adjacent to said continuous voltage terminal being connected over a capacitance to the other terminal of the continuous voltage source and to a group of two fixed contacts of the converter other than said rst mentioned group of xed contacts, and a group of two oscillatory contacts cooperating with said groups of xed contacts and connected to the terminals of an alternating current load shunted by a capacitance.

3. A combination as claimed in claim l, wherein the valves are so directed as to permit an annulment of the accumulated energy of the inductances in currentless in tervals.

4. A combination as claimed ,in claim 2, wherein the valves are so directed as to permit an annulment of the accumulated energy of the inductances Ain cnrrentless intervals.

References Cited in the iiie o f this patent UNITED STATES PATENTS 2,267,260 Alexander Dec. 23, 1941 2,435,515 Rosser Feb. 3, 1948 2,610,231 Wettstein Sept. 9, 1952 2,673,947 Winther Mar. 30, 195.4 

